Data storing disks employ circular or spiral tracks on which the data is recorded. Transducers such as magnetic read/write heads, laser read/write assemblies, and the like must maintain registry with those tracks within relatively close tolerances.
The ideal data storage disk contains a plurality of concentric data tracks that are perfectly circular or spiral in character when established and remain that way indefinitely. Such ideal data tracks are rarely created in disk manufacture and even more rarely remain circular in the contemporary operating environment.
The lack of circularity in the data tracks is known in the industry as runout. If it is too great, the transducer controlling servo system may not have adequate reacting capability to respond so as to maintain data interfacing registry. Further, even though the disk itself might rotate at a constant velocity, the runout forces the servo system to accelerate and decelerate to compensate for the runout and prevent data loss. Thus it is important to determine whether the tracks on the disks are within minimum acceptable tolerances.
In the past, expensive, special purpose equipment was needed to perform the measurement of track runout. It was employed completely independent of the apparatus which interfaces with the disks. For optical disks, such measurements are even more difficult to perform since the tolerances are frequently set in microns. Thus the tolerances of actuator misalignment are likewise measured in microns. The optic disk measurement is currently done by special purpose structures including laser sources, charge coupled detectors, and the like along with appropriate sensors and supporting electronics and hardware, all separate from the disk drive and its electronics.